1. Field of the Invention
This invention relates generally to position determination and tracking systems. More specifically, this invention relates to radio frequency identification (RFID) tag systems, methods and readers. Still more specifically, the present invention relates to RFID tags and tag readers that may utilize a scanning antenna or an electronically steerable passive array antenna and environmental enhancements for significant system improvements.
2. Background Art
Many product-related and service-related industries entail the use and/or sale of large numbers of useful items. In such industries, it may be advantageous to have the ability to monitor the items that are located within a particular range. For example, within a particular store, it may be desirable to determine the presence and position of inventory items located on the shelf, and that are otherwise located in the store.
A device known as an RFID “tag” may be affixed to each item that is to be monitored. The presence of a tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored by devices known as “readers.” A reader may monitor the existence and location of the items having tags affixed thereto through one or more wired or wireless interrogations. Typically, each tag has a unique identification number that the reader uses to identify the particular tag and item.
Currently, available tags and readers have many disadvantages. For instance, currently available tags are relatively expensive compared to many grocery store items. Because large numbers of items may need to be monitored, many tags may be required to track the items. Hence, the cost of each individual tag needs to be minimized. Furthermore, currently available tags consume large amounts of power. These inefficient power schemes also lead to reduced ranges over which readers may communicate with tags in a wireless fashion. Conventional passive RFID tags work by backscattering the incident RF energy with modulation to encode information stored in the tag's memory. Although passive tags are currently used at lower frequencies such as 1.25 MHz and 13 MHz, the inventions described in this application are most relevant to higher frequency tags operating at 433 MHz, 860–950 MHz, or even 2.4 GHz. The reason is simply that directed beams require a large radiating aperture size, typically a minimum of 5 to 10 free-space wavelengths long. One wavelength at 13 MHz is about 75 feet long! However, at 900 MHz, a free-space wavelength is only 13 inches in length, and a practical 900 MHz near field array can occupy a length of less than 8 feet.
As the antennas used with RFID tags readers are typically fixed beam and manually directed, positioning information can be obtained if the tag's position is known, stored in memory, and then relayed to the reader. However, if the tags are moved or are moving or do not possess their position information, their angular position cannot be determined. Thus, there is a need in the art for an RF ID tag system and method that can determine the angular position of the tag relative to the reader.
Conventional UHF frequency (433 MHz and 915 MHz) RFID tags are forward link limited. This means that the forward RF link, from transmit antenna to RFID chip, is the weak link. The receive link, from RFID chip back to the receive antenna, usually has enough margin to work well if the forward link is adequate. So the challenge is to illuminate the RFID tag with adequate power such that its internal charge pump can achieve a given minimum voltage to turn on the tag and begin the handshaking process with the reader. The power supply voltage stored by the tag is the product of incident power level and the dwell time, or time that such incident RF power is available. Hence the longer a tag is illuminated by a reader antenna, the greater is the probability of successfully reading that tag. Consequently, one of many important characteristics of a tracking RFID antenna system is to increase the dwell time that a tag is illuminated by having the reader's transmit beam follow the target tag through its field of view.
Also, when water or other types of liquids are present in the RF environment, the process of reading tags becomes even more exacerbated. High dielectric liquids such as pure water will detune tags that lie in close proximity so that the tag resonates at a lower frequency. Unless the tag is compensated by design, it will become much less efficient when detuned. Also, tagged consumer products that have a high DC conductivity ue to the presence of salts, such as ketchup, soap, or shampoo, will absorb a significant portion of the incident RF power and simply generate heat. In fact, due to the inefficiencies mentioned above, the electromagnetic energy radiated from conventional antennas may not reach the tag's RFID chip with sufficient level, and therefore the tag will not be read.
Thus, among other things, what is needed is a system, method or apparatus capable of increasing the probability of reading RFID tags when such tags are located in hostile environments such as inside cases and inside pallet stacks of RF attenuating consumer product goods.